Water, wastewater and industrial liquid filtration units typically have an underdrain system for supporting filter media such that the filter media is spaced apart from the bottom of the filter. In addition to providing support for the filter media, the underdrain system serves two primary purposes: to collect the filtered water that passes through the media and to uniformly distribute backwash water, backwash air, or a combination of both, across the filter.
Underdrain systems are most often comprised of one of the following: precast concrete blocks, plastic-jacketed concrete blocks, stainless steel pipe laterals or screens, plastic nozzles or all plastic universal blocks. Universal blocks are widely accepted in the potable water market. They are called "universal" because they can provide both backwash water and backwash air for cleaning the filter media without additional piping across the filter floor. Universal blocks also allow for attachment of a retention cap which eliminates the need for gravel. The universal underdrain blocks are placed side-by-side and end-to-end in the bottom of the filter to provide a "false bottom." This type of underdrain block is always grouted to the filter floor.
During backwash operations, air is forced through the air distribution pipes and into the air laterals of the underdrain blocks. The pressurized air can be used as a backwash or combined with water for an air/water backwash. The pressurized air/water combination causes the filtered water to be passed upward through the media with sufficient velocity to prevent filter problems such as mud balls, filter cracking, agglomeration buildup on the media grains, and inactive areas within the filter.
One example of an underdrain system made of precast blocks is U.S. Pat. No. 4,923,606 to Gresh et al. Gresh teaches a precast concrete, plastic jacketed filter underdrain block for downflow or upflow filter units. The plastic jacket/molds are provided with alternating vertical ribs on the outside vertical surfaces. This creates an interlocking system whereby each block may be interconnected to adjacent side blocks. Laterally spaced legs are arranged to define a passage between the legs and extend from one end of the plastic block to the other end.
U.S. Pat. No. 5,160,614 to Brown discloses an underdrain system comprised of a series of modular, interconnected air duct blocks. In this reference, the row of air duct blocks extends transversely to a plurality of laterals. The laterals are defined by the connection of individual underdrain blocks in parallel adjacent rows.
U.S. Pat. No. 5,108,627 to Berkebile et al. disclose a filter underdrain block having a plurality of exterior and interior walls defining a plurality of interior chambers. Reinforcing ribs formed on the exterior surface of the walls may or may not extend the full length of the block. A plurality of spaced perpendicularly extending flange members 52 are provided along each sidewall above the position where the lower ribs terminate. These assist in handling and positioning the blocks along the bottom of the filter. The sidewall also comprises a plurality of indentations to facilitate intimate contact with grouting material when the block is assembled in a filter bed bottom. Col. 6, II. 56-65. The ribs extend around the lower portion of each block and rest on a previously constructed floor in a tank. Col. 7, II 68 -col. 8, I. 1. Reinforcing ribs formed on the exterior surface of the walls may or may not extend the full length of the block. A plurality of spaced perpendicularly extending flange members 52 are provided along each sidewall above the position where the lower ribs 20 terminate. These flange members 52 assist with handling and positioning of the blocks. Col.6, II. 56-61.
U.S. Pat. No. 5,865,999 to Shea et al. disclose a self-supporting underdrain member having a pair of flanges connected to inclined sidewalls. The flanges serve to attach the underdrain block to a filter floor surface. The disclosed underdrain block comprises a pair of generally triangular, transversely extending end walls. No bottom wall is provided in the underdrain block.
Plastic underdrain blocks are advantageously inexpensive to produce and efficient to operate but can cause severe problems. Unlike heavy-duty cement, plastic blocks tend to be buoyant when operating in the backwash mode, especially when high pressure air scouring is combined with water backwash. Typically, underdrain blocks are laid end to end and interlocked to form a 2-20 block lateral string that extends up to 60 feet. The blocks are snapped together end to end to form a tight seal. The lateral strings of blocks are then positioned side to side with an inch or inch and a half space between the parallel strings of blocks.
Grout is used both on the filter floor prior to placement of the blocks and then poured into the spaces between the lateral strings to hold the blocks in place as well as seal any openings or spaces. Leakage can cause the filter system to fail. Grouting helps to secure plastic underdrain blocks to the floor but, under current backwash systems, has not always been sufficient to withstand the pressure of the air/water backwash. Even when grouted to the filter floor, the beginning forces of the air/water backwash exceed the holding forces of the grout and can cause the buoyant underdrain block to pop out of the grout. When the underdrain block or, even worse, a string of interconnected blocks lift off the filter floor, gravel or plate media support systems can be thrown out of balance allowing media to clog the orifices of the piping and support system. If this occurs, the filter is ruined. Media and gravel must be removed and all underdrain blocks pulled out of the grout so that the filter can be rebuilt.
What is needed is a structure applicable to plastic underdrain blocks that provides additional resistance to the pressure of the backwash system. There is a need for a mechanism that retains the advantageous features of a plastic underdrain block, inexpensive manufacturing and efficiency, for example, without the disadvantage of buoyancy that lifts the block from the filter floor thereby disrupting filter operations.